The present invention generally relates to semiconductor manufacturing and more specifically relates to monitoring planarization of a wafer.
The need to remain cost and performance competitive in the production of semiconductor devices has caused continually increasing device density in integrated circuits. To facilitate the increase in device density, new technologies are constantly needed to allow the feature size of semiconductor devices to be reduced. These include improved manufacturing systems and techniques. One notable manufacturing process is that of planarization.
Planarization is the process of removing projections and other imperfections to create a flat planar surface, both locally and globally, and/or the removal of material to create a uniform clearing of a deposited thin film layer on a wafer. Semiconductor wafers are planarized or polished to achieve a substantially smooth cleared patterned surface as part of process steps that create the integrated circuitry or interconnects on the wafer. A considerable amount of effort in the manufacturing of modem complex, high density multilevel interconnects is devoted to the planarization of the individual layers of the interconnect structure. Nonplanar surfaces create poor optical resolution of subsequent photolithography processing steps. Poor optical resolution prohibits the printing of high-density lines. Another problem with nonplanar surface topography is the step coverage of subsequent metalization layers. If a step height is too large there is a serious danger that open circuits will be created. Planar surface layers are required in the fabrication of modern high-density integrated circuits. To this end, chemical-mechanical planarization (CMP) tools have been developed to provide controlled planarization of both structured and unstructured wafers.
CMP consists of a chemical process and a mechanical process acting together, for example, to reduce height variations across a dielectric region, clear metal deposits in damascene processes or remove excess oxide in shallow trench isolation fabrication. The chemical-mechanical process is achieved with a liquid medium containing chemicals that react with the front surface of the wafer when it is mechanically stressed during the planarization process.
In a conventional CMP tool for planarizing a wafer, a wafer is secured in a carrier connected to a shaft. The shaft is typically connected to mechanical means for transporting the wafer between a load or unload station and a position adjacent to a polishing pad mounted to a rigid or flexible platen or supporting surface. Pressure is exerted on the back surface of the wafer by the carrier in order to press the front surface of the wafer against the polishing pad, usually in the presence of slurry. The wafer and/or polishing pad are then moved in relation to each other via motor(s) connected to the shaft and/or supporting surface in order to remove material in a planar manner from the front surface of the wafer.
It is often desirable to monitor the front surface of the wafer during the planarization process. One known method is to use an optical system that monitors the front surface of the wafer in situ by positioning an optical probe under the polishing pad. Laser interferometry, signal template matching and multifrequency analysis techniques, as well as others, are known monitoring methods. The signal from the probe may be transmitted and received through an opening in the polishing pad. The opening in the polishing pad may be filled with an optically transparent material, or xe2x80x9cwindowxe2x80x9d, in order to prevent polishing slurry or other contaminants from being deposited into the probe and obscuring the optical path to the wafer. The data from the optical system is typically analyzed by a control system to determine the current condition of the front surface of the wafer. This allows the system to terminate the planarization process (call end-point) once the front surface of the wafer has reached a desired condition.
A reliable end-point detection system is desirable for maintaining an optimum CMP process. The end-point system detects the point in the planarization process when the overburden being polished is removed everywhere across the wafer. Excessive removal of overburden from the front surface of the wafer, whether a raw sheet film, or an STI, metal or dielectric layer structure on the front wafer surface, may damage the wafer.
One area of particular concern in end-point detection systems is the spectral shape variations. For example, in the spectral shape can be caused by factors such as staining in the pad window or by variations in particle concentrations at the pad-wafer interface. These spectral variations can cause the endpoint signal to drift, and can create false-positives in the end-point detection system, resulting in premature stopping of the CMP process.
What is needed is an improved system for monitoring the front surface of a wafer during a planarization process that compensates for spectral shape variations to more accurately provide end-point detection.
Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.
The present invention provides a system and method for determining endpoint detection in semiconductor wafer planarization. The system and method provide a flexible solution that can compensate for baseline variability induced errors that may otherwise occur in endpoint detection.
The system uses an endpoint detection signal that monitors the optical characteristics of the wafer being planarized. The system and method continue to monitor the detection signal during planarization until it meets endpoint criterion that indicates endpoint completion. When the endpoint criterion is reached, a new snapshot is taken from a previous time period and a new baseline is calculated. The endpoint detection signal is then recalculated based upon the new baseline and the recalculated detection signal is again compared to the endpoint criterion. If the recalculated endpoint detection signal again substantially meets the endpoint criterion then the detection of endpoint is confirmed. If the recalculated detection signal no longer meets the endpoint criterion, the planarization process continues with the new baseline used as the basis for endpoint detection.
The system and method thus allow the use of new baseline values to serve as the basis for endpoint detection. The use of the new baseline serves to compensate for variability in the optical characteristics of the system. For example, it can be used to compensate for variability due to staining of the pad window and/or particle concentration variations at the pad-wafer interface. By compensating for variability, the system and method provides improved endpoint detection in the planarization process.
The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawings.